The present embodiments relate to a shielding for magnetic resonance tomography (MRT).
A shielding is known, for example, from EP 1 746 432 B1.
In the case of MRT devices, resonant or resonantly connected loop antennas of relatively high quality may be used, for example, for sending radio frequency (RF) signals, on one or more “MRT frequencies”, to excite nuclear spins in an object or patient arranged in the examination space. Depending, for example, on the strength of the static “main magnetic field” used in the MRT, the MRT frequencies, for example, may be in the MHz range (e.g., in the range of approximately 100 to 600 MHz). One or a plurality of such RF antennas form an RF coil system for sending the RF excitation signals into the examination space. The same RF coil system may also be used for receiving the RF response signals from the examination space that occur in the course of the MRT examination.
The shielding serves the purpose of shielding the RF coil system from the components of the MRT device lying behind the RF coil system (e.g., radially further outward), which may otherwise adversely affect the electrical properties of the RF loop antennas (e.g., quality). These components include, for example, the gradient coil system (e.g., formed by a plurality of coils) for generating the magnetic field gradient in the examination space that is required for spatial resolution in the MRT examination.
When sending the RF signals, the shielding advantageously reduces losses in the region of the components and structures lying behind and consequently brings about a certain focusing of the sending field into the radially inner examination space.
The shielding according to the prior art cited above has for this purpose a plurality of electrically conductive strips that run in a straight line and parallel to one another and are separated from one another by slits. By using capacitive bridges of the slits, the lowest natural resonance of the strip arrangement below the natural resonance of the excited nuclei or MRT frequencies may be achieved. At the same time, a good shielding effect may be provided.
The provision of slits on the shielding surface or a not overly large width of the individual strips is of importance to the extent that it prevents excessive development of heat of the shielding being caused by eddy currents generated by the gradient coil system in the electrically conductive material. The gradient coil system generates magnetic fields or magnetic field gradients that may change over time with a frequency in the kHz range.
When the known strip shield is used, high-quality RF loop antennas may be provided at a relatively great distance from this shielding in order that the quality of the RF antennas is not influenced so much by the shielding. However, this reduces the space available for the object of examination or the patient.
One possibility of placing a high-quality RF antenna closer to the known strip shield would be to bridge the slits on the shielding surface at suitable points with capacitances such that the mirror currents generated on the loop antennas may flow more or less unhindered on the shielding surface.
With loop antennas of relatively large dimensions, this possibility is not very practicable in the case of the known strip shield, since very many capacitances or capacitor elements would be required for this. Due to the series connection of these capacitances, the individual capacitance values would have to be very great (e.g., in order to allow a mirror current flow that is as unhindered as possible at the MRT frequency concerned). This configuration also allows the formation of undesired resonances, which may likewise reduce the quality of the loop antenna.